Due to stringent exhaust emissions regulations catalyst substrates are being designed and manufactured with thinner walls. To increase the durability and structural integrity of thin walled catalyst substrates zirconia is commonly added to cordierite formulations. The addition of zirconia decreases the porosity of the catalyst substrate, making it prone to experiencing and forming microcracks in its structure. This, in turn, increases the likelihood that the structural integrity of the catalyst substrate may be compromised at high operating temperatures in an exhaust system.
To prevent microcrack formation conventional catalyst substrates having a cordierite formulation are typically coated with alumina and low amounts of platinum, palladium, and/or rhodium. Conventional catalyst substrates also employ high percentages of rare earth metals such as cerium and lanthium, alkaline earth metals such as barium, and transition metals and their oxides such as nickel oxides. Although alumina, when combined with cordierite, does not lower the softening point of cordierite, precious metals, such as the rare earth, alkaline earth and transition metals, when combined with cordierite, all lower the softening and melting point of cordierite.
One drawback associated with existing catalyst substrates is the increased thermal expansion and conductivity of the catalyst substrate as a result of excessive precious metal-cordierite material blends. Another recognized drawback is that alumina fills the microcracks in the cordierite when applied to the catalyst substrate. As the catalyst substrate's temperature increases, alumina will expand within the microcracks and further compromise the structural integrity of the catalyst substrate. Yet another recognized drawback is that precious metal coatings are commonly applied with an acidic solution, which etches the cordierite surface, and can compromise the structural integrity of the catalyst substrate even further.
Consequently, there is a need for a catalyst substrate having improved thermal durability without the above-mentioned drawbacks.